1. Field of the Invention
The present invention relates to a persistent current circuit switch used to operate a super conductive coil by a permanent electric current, and more particularly to a persistent current circuit switch in which a pair of electrodes are oppositely arranged so as to contact and separate from each other. Contacts of the electrodes are brought into line contact or dot series contact with each other.
2. Description of the Related Art
In a persistent current circuit switch that short-circuits both ends of a super conductive coil, it is required that contact resistance value in a turning-on (closed) state of the persistent current circuit switch be very small. A clean contact surface can be obtained in a persistent current circuit switch of vacuum type having electrodes arranged in a vacuum since impurities, an oxide film, etc. are not attached to the contact surface.
For example, a persistent current circuit switch disclosed in Japanese Examined Patent Application No. Sho 56-16932 is conventionally known as the persistent current circuit switch of vacuum type. FIG. 17 shows this persistent current circuit switch. FIG. 18 is a cross-sectional view of a portion taken along the broken line 18--18 shown in FIG. 17.
In FIG. 17, reference numerals 1, 2 and 3 respectively denote a movable electrode, a super conductive metallic member on the side of the movable electrode, and a superconductive metallic member on the side of a fixed electrode. Reference numerals 4, 5 and 6 respectively denote the fixed electrode, a metallic cover on the fixture side, and a movable electrode joining member. Reference numerals 7, 8 and 9 respectively denote bellows, a metallic cover on the movable side, and a metallic sleeve on the movable side. Reference numerals 10 and 11 respectively denote an insulating sleeve and a metallic sleeve on the fixture side.
FIG. 19 is a view showing characteristics of a contact of this conventional persistent current circuit switch. FIG. 20 is a view showing contact traces of the contact of the conventional persistent current circuit switch.
An operation of this switch will next be explained.
In FIGS. 17 and 18, each of the movable electrode 1 and the fixed electrode 4 is constructed by, for example, a fine metallic material such as copper and silver. Superconductive metallic members 2, 3 each constructed by, for example, an Nb-Ti alloy having a columnar shape are respectively buried in central portions of the movable electrode 1 and the fixed electrode 4 along a flow direction of an electric current. Closing and opening operations of the movable electrode 1 are performed by a not-shown driving mechanism. When the persistent current circuit switch is turned on (closed), the movable electrode 1 and the fixed electrode 4 are closed so that the superconductive metallic members 2 and 3 are brought into contact with each other and a superconductive contact is formed.
Reference numeral 7 in FIG. 17 denotes bellows. One end of the bellows 7 is airtightly and fixedly attached to the movable electrode 1 via the movable electrode joining member 6. The other end of the bellows 7 is airtightly and fixedly attached to the insulating sleeve 10 made of a ceramic through the movable side metallic cover 8 and the movable side metallic sleeve 9. On the other hand, another bellows on the fixed electrode 4 side is also airtightly and fixedly attached to the insulating sleeve 10 through the fixing side metallic cover 5 and the fixing side metallic sleeve 11. The air in the interior of an airtight container is removed to form a vacuum.
It is necessary to place the persistent current circuit switch under a very low temperature state to maintain each of the superconductive metallic members 2, 3 in a superconductive state. Accordingly, the persistent current circuit switch is cooled by not-shown liquid helium, a refrigerator, etc.
FIG. 19 shows characteristics of a flowing electric current and a contact resistance value between the electrodes at a closing time in the persistent current circuit switch in which the superconductive metallic member 2 is 20 mm in diameter and a planned flowing electric current value is set to I=1000 [A] in the above conventional structure.
According to the drawing, when the flowing electric current I is gradually increased from 0 [A] at the closing time of the switch, the contact resistance value R is maintained in a super low resistance state of 2.times.10.sup.-9 [.OMEGA.] since the switch is in the superconductive state. However, the superconductive contact is suddenly quenched (the superconductive state is broken) near I=300 [A], and the contact resistance value changes to a state of R=2.times.10.sup.-5 [.OMEGA.].
FIG. 20 shows a contacting situation of contacts of the superconductive metallic member 2 on the side of the movable electrode 1 and the superconductive metallic member 3 on the side of the fixed electrode 4 after the closing and opening operations are performed about ten times in the above conventional structure. Reference numeral 15a denotes contact traces left on a surface of the superconductive metallic member 3 on the side of the fixed electrode 4. These traces should be wide traces left along the entire circumference of the superconductive metallic member 3 when the superconductive metallic members 2 and 3 are ideally brought into contact with each other.
In the conventional persistent current circuit switch, as mentioned above, though a very small contact resistance value is obtained when the electric current value is low, a contact face is quenched when the electric current value is higher than the planned electric current value. Therefore, when the persistent current circuit switch is actually used, a problem exists in that three persistent current circuit switches or more are connected in parallel to each other and as large a persistent current circuit switch as one having a planned flowing electric current value I=3000 [A] is used, etc. so that a device is complicated and large-sized.